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Abstract Estimation of soil sorptivity (S) and hydraulic conductivity (K) is fundamental to model the water infiltration into the soil. This process can be affected by soil water repellency, which is defined as a reduction in soil wettability due to coating of soil particles by hydrophobic substances. Unlike to wettable soils, this phenomenon can generate infiltration curves with double-slope shape: a transient infiltration curve followed by a steady-state section. Because the topsoil final volumetric water content (θ 1) of the transient phase of the double-slope curve is not a measurable data, in principle, the standard model based on the Haverkamp et al. (1994) model cannot be used to estimate S and K. This work presents two different approaches based on the Haverkamp et al. (1994) equation, which allow estimating S and K from the first phase of a double-slope infiltration curve, when θ 1 data are not available. The methods, which are based on the analysis of both short-medium time transient infiltration curve (Tr) and the combination of both short-medium transient and steady-state infiltration steps (Mx), were applied on 20 soils affected by different degrees of water repellency. The Haverkamp et al. (1994) model was also valid for infiltration curves measured on hydrophobic soils, and the final volumetric water content was not an essential data to estimate K and S. Although the steady-state infiltration rate (q 1) calculated with Mx was about 26% larger than that estimated with Tr , comparable K and S values were obtained with both methods. Overall, a large dispersion on the estimate of θ 1 was observed with both methods. The gravimetric time, t grav , estimated in the studied soils was low, <500 s. While the Mx method required simpler numerical calculus, Tr looked like to be more robust and less subjective. Highlights • This work analyzes the first part of a bi-slope infiltration curve affected by SWR. • A first method (Tr) estimates K and S from the transient infiltration curve. • The second one (Mx) combines short-transient and steady-state infiltration analysis. • The methods were applied on 20 soils affected by SWR. • Mx requires less numerical calculus, but Tr was more robust and less subjective. [ABSTRACT FROM AUTHOR]

Highlights • This work analyses the influence of the β on the estimate of K and S. • Early infiltration times (i.e. 100 s) promoted good estimations for S. • Longer infiltrations (i.e. 1.000 s) were required to estimate accurately K s. • Only very long infiltration (i.e. 10.000 s) allowed defining the actual β value. • S and K s can be accurately estimated using a constant β. Abstract Soil water infiltration can be described with the quasi-analytical Haverkamp et al. (1994) equation, defined by the hydraulic conductivity (K s), sorptivity (S) and the β parameter. K s and S are commonly estimated from the transient cumulative infiltration curve, using a constant β value. The objective of this work was to study the influence of β on the estimation of K s and S. The study was first performed on synthetic 1-D infiltration curves generated at different infiltration times for loam sandy, loamy and silty soils, and next extrapolated to a 3-D loam synthetic soil and on 10 infiltrations curves measured on the field with a disc infiltrometer on different types of soils. The infiltration measurements lasted between 600 and 900 s. The results showed that, while early infiltration times (i.e. 100 s) promoted good estimations for S , longer infiltrations (i.e. 1.000 s) were required to estimate accurately K s. Only very long infiltration (i.e. 10.000 s) allowed defining the actual β value. A similar behaviour was observed for the 3-D infiltration measurements. Except for β , significant relationships (R2 = 0.99) were obtained between the K s and S of the three theoretical soils and the corresponding values calculated by optimizing the three hydraulic parameters on 2.000 s 1-D infiltration curves. The large confidence interval observed in β (between 0.3 and 2.0) resulted from the fact that β had a small effect on the infiltration curve. A significant relationship (R2 = 0.99) was also obtained between the K s and S optimized from the experimental curves using a β = 1.1 and the corresponding values obtained by simultaneous optimization of the three hydraulic parameters. These results demonstrated that S and K s can be accurately estimated using a constant β and that downward infiltration is not an appropriate procedure to estimate β. [ABSTRACT FROM AUTHOR]

The water retention curve ( θ ( h )), which defines the relationship between the volumetric water content ( θ ) and the matric potential ( h ), is of paramount importance to characterize the hydraulic behaviour of soils. Because current methods to estimate θ ( h ) are, in general, tedious and time consuming, alternative procedures to determine θ ( h ) are needed. Using an upward infiltration curve, the main objective of this work is to present a method to determine the parameters of the van Genuchten (1980) water retention curve ( α and n ) from the sorptivity ( S ) and the β parameter defined in the 1D infiltration equation proposed by Haverkamp et al. (1994). The first specific objective is to present an equation, based on the Haverkamp et al. (1994) analysis, which allows describing an upward infiltration process. Secondary, assuming a known saturated hydraulic conductivity, K s , calculated on a finite soil column by the Darcy’s law, a numerical procedure to calculate S and β by the inverse analysis of an exfiltration curve is presented. Finally, the α and n values are numerically calculated from K s , S and β . To accomplish the first specific objective, cumulative upward infiltration curves simulated with HYDRUS-1D for sand, loam, silt and clay soils were compared to those calculated with the proposed equation, after applying the corresponding β and S calculated from the theoretical K s , α and n . The same curves were used to: (i) study the influence of the exfiltration time on S and β estimations, (ii) evaluate the limits of the inverse analysis, and (iii) validate the feasibility of the method to estimate α and n . Next, the θ ( h ) parameters estimated with the numerical method on experimental soils were compared to those obtained with pressure cells. The results showed that the upward infiltration curve could be correctly described by the modified Haverkamp et al. (1994) equation. While S was only affected by early-time exfiltration data, the β parameter had a significant influence on the long-time exfiltration curve, which accuracy increased with time. The 1D infiltration model was only suitable for β < 1.7 (sand, loam and silt). After omitting the clay soil, an excellent relationship (R 2 = 0.99, p < 0.005) was observed between the theoretical α and n values of the synthetic soils and those estimated from the inverse analysis. Consistent results, with a significant relationship (p < 0.001) between the n values estimated with the pressure cell and the upward infiltration analysis, were also obtained on the experimental soils. [ABSTRACT FROM AUTHOR]

Summary Many gypsiferous soils occur in arid lands, where the water retention capacity of the soil is vital to plant life and crop production. This study investigated the effect of gypsum content on the gravimetric soil water retention curve (WRC). We analyzed calcium carbonate equivalent (CCE), equivalent gypsum content (EG), soil organic carbon content (SOC), and electrical conductivity of 43 samples collected from various horizons in soils in the Ebro Valley, NE Spain. The WRC of the fine earth was determined using the pressure-plate method (pressure heads = 0, −33, −100, −200, −500, and −1500 kPa), and the gravimetric water retention curves were fitted to the unimodal van Genuchten function. Soil gypsum content had a significant effect on water retention. Soils that had high gypsum content made WRC with higher water retention at near saturation conditions, and steeper WRC slopes. The EG threshold at which gypsum content had an effect on WRC was about 40%, and EG was positively and negatively correlated with the α and n parameters of the WRC, respectively. [ABSTRACT FROM AUTHOR]

• Effect of grazing on soil hydraulic properties of gypseous soils were studied. • Three grazing intensities, High, Medium and Low, were compared. • Soil surface S and K c and subsurface ρ b , K s , θ s , α and n were measured. • H presented the highest ρ b and the lowest K c values. • In semiarid gypseous soils M did not have a negative effect of soil properties. Although is well known that grazing has a significant effect on the soil hydraulic properties and through them on the pasture productivity in semi-arid regions, there is still little data about the effect of this management on soils with high gypsum content. This work compares the effect of three increasing grazing intensities (low, medium, and high) on the hydraulic properties of the upper soil layers, measured on two areas with increasing gypsum content (56 and 73%, respectively) located in a semi-arid region of the Middle Ebro Valley (NE, Spain). 90 soil samplings were taken on bare soil surface areas. The measured soil parameters included the saturated sorptivity (S c) and hydraulic conductivity (K c) of the soil surface layer and the soil bulk density (ρ b), saturated hydraulic conductivity (K s), saturated volumetric water content, θ s , and α and n parameters of the water retention curve of the 1–8 cm depth soil layer. Increasing gypsum content increased the soil water infiltration but did not affect the water retention capacity and total porosity. All studied gypseous soils presented low water retention. High grazing intensities decreased the soil water infiltration. While soils with high gypsum content tended to attenuate the effect of the livestock trampling on ρ b , a significant influence of grazing on ρ b was observed in the fields with moderate soil gypsum content. No effect of livestock trampling on the soil water retention characteristics was observed. Moderate grazing, which did not show negative effects on the soil hydro-physical properties, could be an acceptable management for soil conservation in these semiarid regions of soils with high gypsum content. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a new type of pressure cell associated with a zigzag-shaped time domain reflectometry (TDR) probe for determining the soil-water retention (θ(ψ)) curve of disturbed thin soil samples. The pressure cell, designed for pressures ranging between 0 and −500kPa, consisted of a zigzag copper rod (150-mm long, 2mm in diameter) vertically installed in a clear plastic cylinder (60-mm high, 50mm in internal diameter) with six vertical copper rods (60-mm long, 2mm in diameter) arranged around the inner wall of the plastic cylinder. The cylinder was closed at the base with a nylon cloth and placed on a porous ceramic disc. The inner rod and the six-rod grille of the cell were connected respectively to the inner and outer conductors of a coaxial cable. The results showed that the correlation between the apparent dielectric constant measured with a standard three-rod TDR probe and the zigzag-shaped TDR probe, both immersed in five different non-conductive fluids, was excellent (R 2 =0.99). On the other hand, the volumetric water content measured with the TDR probe of the pressure cell filled up with sand, 2-mm sieved loam and clay-loam soils was highly correlated to the corresponding values calculated from the gravimetric water content and the soil bulk density (R 2 =0.97; RMSE=2.32×10−2). The parameters of the θ(ψ) curves measured for these three different soils with the TDR-pressure cell were within the range of values found in the literature. The cell was also used to study the θ(ψ) changes of a 2–4-mm sample of loam soil aggregates after a slow and a fast-wetting process. While negligible changes in both the soil structure and θ(ψ) were observed following slow wetting, fast wetting resulted in disintegration of aggregates and drastic changes in the shape of the θ(ψ) curve. [Copyright &y& Elsevier]

Summary Based on the analysis of Haverkamp et al. (1994), this paper presents a new technique to estimate the soil hydraulic properties (sorptivity, S , and hydraulic conductivity, K ) from the full-time cumulative infiltration curves. The proposed method, which will be named as the Numerical Solution of the Haverkamp equation (NSH), was validated on 12 synthetic soils simulated with HYDRUS-3D. The K values used to simulate the synthetic curves were compared to those estimated with the NSH method. A procedure to detect and remove the effect of the contact sand layer on the cumulative infiltration curve was also developed. A sensitivity analysis was performed using the water level measurement as uncertainty source and the procedure was evaluated considering different infiltration times and data noise (e.g. air-bubbling in the infiltrometer). The good correlation between the K used in HYDRUS-3D to model the infiltration curves and those obtained by the NSH method ( R 2 = 0.98) indicates this technique is robust enough to estimate the soil hydraulic conductivity from complete infiltration curves. The numerical procedure to detect and remove the influence of the contact sand layer on the K and S estimates resulted to be robust and efficient. A negative effect of the curve infiltration noise on the K estimate was observed. The results showed that infiltration time was an important factor to estimate K . Smaller values of K or lower uncertainty required longer infiltration times. In a second step, the technique was tested in field conditions on 266 different soils at saturation conditions, using a 10 cm diameter disc infiltrometer. The NSH method was compared to the standard differentiated linearization procedure (DL), which estimates the hydraulic parameters using the simplified Haverkamp et al. (1994) equation, valid only for short to medium times. Compared to DL, NSH was considerably less affected by the infiltration bubbling and the contact sand layer, and allowed more robust estimates of K and S . Although comparable S values were obtained with both methods, the NSH technique, which is not limited to short times, resulted in more accurate and robust estimates for K . This paper demonstrates the NSH method is a significant advance to estimate of the soil hydraulic properties from the transient water flow. [ABSTRACT FROM AUTHOR]